Always decode 3 or 4 channel PNG images.

The bug @HayesGordon ran into with Unity was because we were trying to decode grayscale images to a single-channel image and then expanding those to 4 channels as the Rive Renderer expects RGBA. That was padding missing channel data with 255, which is why we were getting Cyan (R would be read, GBA were filled with 255).

Instead, we make the Bitmap library only work with RGB and RGBA and we let libpng do the expansion for us.

Glorious:
<img width="417" alt="CleanShot 2024-03-08 at 21 28 21@2x" src="https://github.com/rive-app/rive/assets/454182/326e537b-15ad-4914-acb7-a85ddc42c88c">

Diffs=
09feaccb0 Always decode 3 or 4 channel PNG images. (#6818)

Co-authored-by: Luigi Rosso <[email protected]>

.rive_head

@@ -1 +1 @@
-7cb7eb8122d3625aad2c6032615006cc8f5383e8
+09feaccb0c5bb3a2f88c23aca6c669d79eee4428

decoders/include/rive/decoders/bitmap_decoder.hpp

@@ -13,7 +13,6 @@ class Bitmap
 public:
     enum class PixelFormat : uint8_t
     {
-        R,
         RGB,
         RGBA
     };

decoders/src/bitmap_decoder.cpp

@@ -23,8 +23,6 @@ size_t Bitmap::bytesPerPixel(PixelFormat format) const
 {
     switch (format)
     {
-        case PixelFormat::R:
-            return 1;
         case PixelFormat::RGB:
             return 3;
         case PixelFormat::RGBA:

decoders/src/decode_png.cpp

@@ -73,48 +73,42 @@ std::unique_ptr<Bitmap> DecodePng(const uint8_t bytes[], size_t byteCount)
                  NULL,
                  NULL);
 
-    png_set_strip_16(png_ptr);
-
-    int bitDepth = 0;
-    if (color_type == PNG_COLOR_TYPE_PALETTE || color_type == PNG_COLOR_TYPE_RGB)
+    if (color_type == PNG_COLOR_TYPE_PALETTE ||
+        (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8))
     {
+        // expand to 3 or 4 channels
         png_set_expand(png_ptr);
-        bitDepth = 24;
-        if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
-        {
-            png_set_expand(png_ptr);
-            bitDepth += 8;
-        }
     }
-    else if (color_type == PNG_COLOR_TYPE_GRAY)
+
+    png_bytep trns = 0;
+    int trnsCount = 0;
+    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
     {
+        png_get_tRNS(png_ptr, info_ptr, &trns, &trnsCount, 0);
         png_set_expand(png_ptr);
-        bitDepth = 8;
     }
-    else if (color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+
+    if (bit_depth == 16)
     {
-        png_set_expand(png_ptr);
-        png_set_gray_to_rgb(png_ptr);
-        bitDepth = 32;
+        png_set_strip_16(png_ptr);
     }
-    else if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
+
+    if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
     {
-        png_set_expand(png_ptr);
-        bitDepth = 32;
+        png_set_gray_to_rgb(png_ptr);
     }
 
-    int pixelBytes = bitDepth / 8;
-    auto pixelBuffer = new uint8_t[width * height * pixelBytes];
+    png_read_update_info(png_ptr, info_ptr);
+    uint8_t channels = png_get_channels(png_ptr, info_ptr);
+
+    auto pixelBuffer = new uint8_t[width * height * channels];
 
     png_bytep* row_pointers = new png_bytep[height];
 
     for (unsigned row = 0; row < height; row++)
     {
         unsigned int rIndex = row;
-        // if (flipY) {
-        //     rIndex = height - row - 1;
-        // }
-        row_pointers[rIndex] = pixelBuffer + (row * (width * pixelBytes));
+        row_pointers[rIndex] = pixelBuffer + (row * (width * channels));
     }
     png_read_image(png_ptr, row_pointers);
     png_read_end(png_ptr, info_ptr);
@@ -124,18 +118,15 @@ std::unique_ptr<Bitmap> DecodePng(const uint8_t bytes[], size_t byteCount)
     delete[] row_pointers;
 
     Bitmap::PixelFormat pixelFormat;
-    assert(bitDepth == 32 || bitDepth == 24 || bitDepth == 8);
-    switch (bitDepth)
+    assert(channels == 3 || channels == 4);
+    switch (channels)
     {
-        case 32:
+        case 4:
             pixelFormat = Bitmap::PixelFormat::RGBA;
             break;
-        case 24:
+        case 3:
             pixelFormat = Bitmap::PixelFormat::RGB;
             break;
-        case 8:
-            pixelFormat = Bitmap::PixelFormat::R;
-            break;
     }
     return std::make_unique<Bitmap>(width, height, pixelFormat, pixelBuffer);
 }